Background: We compared the magnetic resonance imaging (MRI) features between Japanese and Caucasian patients with multiple sclerosis (MS), and identified the relationships between MRI features and disability. Methods: From the baseline data of phase II fingolimod trials, 95 Japanese and 246 Caucasian relapsing-remitting MS patients were enrolled. The number, volume, and distribution of brain MRI lesions were evaluated using T2-weighted (T2W) images. Cross-sectional total normalized brain volume (NBV), normalized cortical gray matter volume, normalized deep gray matter volume (NDGMV), normalized white matter volume (NWMV), and normalized thalamic volume were measured. Results: Japanese patients had significantly lower Expanded Disability Status Scale (EDSS) scores than Caucasian patients (mean 2.0 vs. 2.3, p = 0.008), despite a similar disease duration. Japanese patients showed a trend towards fewer T2W-lesions (median 50 vs. 65, p = 0.08) and significantly lower frequencies of cerebellar and parietal lobe lesions (p = 0.02 for both) than Caucasian patients. There were no differences in T2W-lesion volume between races, whereas Japanese patients had a significantly larger T2W-lesion volume per lesion compared with Caucasian patients (median 140 mm3 vs. 85 mm3, p < 0.0001). T2W-lesion volumes were positively correlated with EDSS scores in Japanese patients (p < 0.0001). In both races, NBV, normalized cortical gray matter volume, NDGMV, and thalamic volume were negatively correlated with disease duration and EDSS scores (p < 0.01 for all). NWMV was negatively correlated with disease duration and EDSS scores only in Caucasian patients (p = 0.03 and p = 0.004, respectively). NBV, NDGMV, NWMV, and thalamic volume were consistently smaller in Japanese compared with Caucasian patients throughout the entire examined disease duration (p = 0.046, p = 0.01, p = 0.005, and p = 0.04, respectively). Japanese patients had a significantly faster reduction in NDGMV (p = 0.001), particularly for thalamic volume (p = 0.001), with disease duration compared with Caucasian patients. Conclusions: Gray matter atrophy is a common denominator for disability in Japanese and Caucasian patients. Additional contributory factors for disability include T2W-lesion volume in Japanese patients and white matter atrophy in Caucasian patients. Less frequent parietal and cerebellar involvement with fewer T2W-lesions may underlie milder disability in Japanese patients.
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